1. Field of the Invention
The present invention relates to a balanced-to-unbalanced transformer including a +90° phase shifter and a −90° phase shifter and to an amplifier circuit module including the balanced-to-unbalanced transformer.
2. Description of the Related Art
A balanced-to-unbalanced transformer including a +90° phase shifter and a −90° phase shifter is disclosed in Japanese Unexamined Patent Application Publication No. 2005-198167. FIG. 1 is a circuit diagram illustrating an example of the transformer. Referring to FIG. 1, a high-pass filter 120 is a +90° phase shifter that has a cutoff frequency lower a design frequency f0 and that is designed so that an output phase with respect to an input phase is +90° at the design frequency f0. A low-pass filter 130 is a −90° phase shifter that has a cutoff frequency higher than the design frequency f0 and that is designed so that an output phase with respect to an input phase is −90° at the design frequency f0. With this configuration, a balanced signal and an unbalanced signal having inverted polarities are mutually transformed.
The balanced-to-unbalanced transformer including the high-pass filter and the low-pass filter according to Japanese Unexamined Patent Application Publication No. 2005-198167 is often used in combination with an amplifier that is used in a mobile phone terminal, for example, and that includes a balanced terminal. In such a use, a circuit configuration of applying a bias voltage to the amplifier via the balanced-to-unbalanced transformer may be used to miniaturize a mounting board.
However, in the circuit illustrated in FIG. 1, bias power supplies are provided for respective balanced terminals P2 and P3, which increases the size of the circuit. Also, a plurality of power supply circuits are required, which increases the cost of the device. Furthermore, in the circuit configuration according to Japanese Unexamined Patent Application Publication No. 2005-198167, when a DC power supply functioning as a bias power supply is disposed on the terminal P3 side, a bias voltage (current) flows to the unbalanced terminal P1 side other than to the terminal P3 side.